Hydrogen absorbing electrode, nickel electrode and alkaline storage battery

ABSTRACT

This invention relates to a hydrogen absorbing electrode, in which a rare earth element having a basicity weaker than that of La is mixed to a hydrogen absorbing alloy or contained in it for serving as a component element. The invention relates to a nickel electrode, in which a rare earth element is mixed to a nickel hydroxide or contained in it as a solid solution. The invention further relates to an alkaline storage battery, in which a rare earth element is coated on a surface of a nickel electrode or a surface of a separator.

TECHNICAL FIELD

This invention relates to a hydrogen absorbing electrode and a nickelelectrode for use in an alkaline storage battery and further to thealkaline storage battery.

BACKGROUND ART

In recent years, a nickel hydride storage battery has attracted publicattention because of its high energy density and low pollution ascompared with a conventional nickel-cadmium storage battery. Further,many studies and developments have been made on it for use as powersources for portable equipment and electric motor etc. In the nickelhydride storage battery, a hydrogen absorbing electrode using a hydrogenabsorbing alloy, which can absorb and release hydrogen in a reversiblemanner, is used for a negative electrode and a nickel electrodeutilizing a nickel hydroxide as an active material is used for apositive electrode.

This nickel hydride storage battery is used for a sealed-type storagebattery. In this case, its negative electrode capacity is made largerthan its positive electrode capacity, so that oxygen gas produced fromthe positive electrode at time of over-charging is consumed by thenegative electrode. Thereby, a sealed system is formed.

However, when charge/discharge cycles are repeated, a gas absorbingperformance and a charge efficiency at the negative electrode arereduced due to oxidation etc. of the hydrogen absorbing alloy. For thisreason, there have been such problems as a rise in battery insidepressure, an increase in internal resistance due to loss of electrolyte,and a lowering of battery performance.

It is well known that a surface of the hydrogen absorbing electrodedeteriorated by oxidation etc. is covered with a lot of needle-likeproducts and these needle-like products are composed of hydroxides ofrare earth elements etc. These needle-like products are produced in sucha manner that rare earth element forming principal component element ofthe hydrogen absorbing alloy is eluted and deposited. The products growwith an increase in charge/discharge cycle number, reduce a conductivityand lessen an utilization of negative electrode capacity.

As a means for solving such a phenomenon, a method is employed nowwherein corrosion resistance is improved by decreasing a quantity of Lain the hydrogen absorbing alloy. This is because La is strongest in itsbasicity among the rare earth element forming the component element ofhydrogen absorbing alloy. However, this method has included suchproblems as a small effect of the improvement in corrosion resistanceand a decrease in discharge capacity.

On the other hand, alkaline storage batteries such as a nickel hydridestorage battery, a nickel-zinc storage battery and a nickel-cadmiumstorage battery are frequently used under high temperature conditionsbecause they are installed particularly in various equipments.Therefore, it is required to improve an utilization of active materialunder high temperature. However, a charge efficiency of the nickelelectrode lowers when its temperature rises, so that the utilization ofactive material decreases, the electrolyte is exhausted by gas generatedand a battery life is shortened.

In order to improve the utilization of active material at hightemperature, such methods are generally used as [1] a method forchanging a component of electrolyte, for example, a method whereinlithium hydroxide aqueous solution is added to potassium hydroxideaqueous solution, and [2] a method wherein a quantity of Co existing asa solid solution in crystal of nickel hydroxide is increased. However,the method [1] includes such problems as a reduction in the utilizationof active material at low temperature and a lowering of high-ratedischarge performance. And, when the quantity of Co is increasedexcessively, the method [2] includes such troubles as a lowering ofdischarge voltage and an increase in manufacturing cost.

DISCLOSURE OF THE INVENTION

A first object of this application is to provide a hydrogen absorbingelectrode which can control an increase in a battery internal pressureand offer an excellent charge/discharge cycle characteristic.

A second object of this application is to provide a nickel electrodewhich includes a high utilization in a wide temperature region;especially in a high temperature region, and has a stabilized capacitycharacteristic.

A third object of this application is to provide an alkaline storagebattery which includes a stabilized capacity characteristic and anexcellent cycle characteristic.

A first invention of this application relates to a hydrogen absorbingelectrode. In other words, the invention is characterized by that, in ahydrogen absorbing electrode using a hydrogen absorbing alloy capable ofabsorbing and releasing hydrogen, simple substance or compound of rareearth element which is weaker in its basicity than that of lanthanum ismixed to the hydrogen absorbing alloy.

In this invention, since the mixed rare earth element is weaker in itsbasicity than that of La, a protective coat under passive state which ismore stable in alkaline electrolyte than La is formed on a surface ofthe hydrogen absorbing alloy. For this reason, elusion of the rare earthelement from inside of the hydrogen absorbing alloy is controlled andthe charge/discharge cycle increases. In addition, since the rare earthelement is mixed, the protective coat is formed at any time even on anew alloy surface created when the hydrogen absorbing alloy cracks inconnection with the charge/discharge cycle. Thereby, the above-mentionedeffect is brought about preferably. According to this invention, sincethe corrosion resistance of electrode can be improved without changingthe component element of the hydrogen absorbing alloy, the invention isfavorable in terms of the alloy capacity and cost as compared with acase where the component element is changed.

In this invention, it is preferable to employ the followingconstructions.

(1) The rare earth element is composed at least one kind of cerium,erbium, gadolinium and ytterbium.

In this construction, Er and Yb are specially excellent in terms of thecorrosion resistance. Ce is a trivalent or tetravalent metal anddifferent in its quality from another rare earth elements in terms of apoint that its form is apt to change. Therefore, when Ce is used,electrode characteristics such as a charge/discharge efficiency and agas absorbing ability by catalytic action are improved.

(2) The compound of rare earth element is an oxide, a hydroxide or ahalogenide.

In this construction, a stability and a catalytic ability are improvedand the cost problem becomes advantageous too.

A second invention of this application also relates to a hydrogenabsorbing electrode. In other words, the invention is characterized bythat, in a hydrogen absorbing electrode using a hydrogen absorbing alloycapable of absorbing and releasing hydrogen, a rare earth element whichis weaker in its basicity than that of lanthanum is contained in thehydrogen absorbing alloy as its component element.

Even in this invention, a protective coat under passive state is formedon a surface of the hydrogen absorbing alloy by the contained rare earthelement, in the same way as the above first invention. Therefore, thecharge/discharge cycle characteristic is improved.

In this invention, it is preferable to employ the followingconstructions.

(1) The rare earth element is composed at least one kind of samarium,gadolinium, terbium, dysprosium, holmium, erbium, thulium and ytterbium.

In this construction, each element of Gd, Tb, Dy, Ho, Er, Tm, Tb and Smis effective, and especially Er and Yb are effective. A small amount ofaddition of each can provide an excellent corrosion resistance.

(2) The hydrogen absorbing alloy is previously subjected to a dippingtreatment using alkaline aqueous solution or weak acidic aqueoussolution.

In this construction, the rare earth element on the surface of hydrogenabsorbing alloy is previously removed. Therefore, the rare earth elementis prevented from being eluted and deposited at time of charging anddischarging in the initial activation, the rare earth element isprevented from being eluted even at time of charging and dischargingafter the activation, so that the charge/discharge cycle characteristicis improved. Especially, when the alloy is subjected to the dippingtreatment using a weak acidic solution or buffer solution, an excellentcapacity can be provided from an initial stage.

A third invention of this application also relates to a nickelelectrode. The invention is characterized by that, in a nickel electrodeusing a nickel hydroxide as its active material, simple substance orcompound of the rare earth element is mixed to the active material.

In this invention, Yb or Yb compound for example, is deposited as astable hydroxide in a strong alkali. This hydroxide raises an oxygenevolution overvoltage to offer an effect for preventing decomposition ofthe electrolyte, so that the charge efficiency of nickel electrode athigh temperature is improved. In addition, this hydroxide forms theprotective coat under passive state on the surface of the hydrogenabsorbing alloy, even in case of the hydrogen absorbing electrode.Therefore, the elusion of rare earth element from the inside of hydrogenabsorbing alloy is controlled. Accordingly, the hydrogen absorbing alloyis prevented from being corroded and a service life of the hydrogenabsorbing electrode forming the negative electrode is prolonged.

In this invention, it is preferable to employ the followingconstruction.

(1) The rare earth element is composed of ytterbium.

(2) The simple substance or compound of rare earth element exists undera state free from the active material.

According to this construction, the characteristics of rare earthelement are maintained and conveniency of manufacture is improved.

(3) The compound of rare earth element is composed of an oxide, ahydroxide or a halogenide.

In this construction, a stability in alkaline and a catalytic abilityare improved and the cost problem becomes advantageous too.

(4) At least one kind of cobalt, zinc, cadmium and magnesium iscontained in the nickel hydroxide as a solid solution. The rare earthelement is composed at least one kind of yttrium, holmium, erbium,thulium, ytterbium, europium and lutetium.

In this construction, it is intended to prolong the service life ofbattery by adding Cd, Zn or Mg to the nickel hydroxide as a solidsolution. In addition, it is intended to improve the charge efficiencyby adding Co as a solid solution, and to further improve the chargeefficiency especially at high temperature by adding the mixed rare earthelement. Namely, when Cd, Zn or Mg is added into the nickel hydroxide asa solid solution, an electrode swelling can be controlled. Accordingly,a phenomenon wherein the electrolyte in a separator is exhausted due toa pressure on the separator caused by the electrode swelling, iscontrolled so that the battery service life can be prolonged.

A potential difference (η value) between an oxidation potential and anoxygen evolution potential of the nickel hydroxide correlates with thecharge efficiency, and the charge efficiency has a tendency to becomelarge with an increase in the η value. Since the oxidation potential ofthe nickel hydroxide shifts to a base side when Co is added as a solidsolution to the nickel hydroxide, the η value is increased and thecharge efficiency at high temperature is improved.

On the other hand; Y, Ho, Er, Tm, Yb and Lu etc. have an effect to shiftthe oxygen evolution potential to a more noble potential. Accordingly,the charge efficiency is improved further when these rare earth elementsare added. This effect owes to a synergetic action between Co and therare earth elements. The foregoing rare earth elements have an effect toconsiderably increase the charge efficiency at high temperature ascompared with La and Ce etc. Especially this effect is remarkable in Yboxide, Er oxide and mixed rare earth element oxide containing Yb etc.Even when the solid solution element is Cd, Zn or Mg, the chargeefficiency improving effect is brought about by the foregoing rare earthelements.

In the foregoing invention (4), it is preferable to set an internal porevolume of the nickel hydroxide to a value smaller than or equal to 0.1ml/g. By using such a high-density nickel hydroxide, the high-ratedischarge characteristic is improved and the nickel electrode stable athigh temperature and having a large capacity can be obtained.

A fourth invention of this application also relates to a nickelelectrode. The invention is characterized by that, in a nickel electrodeusing a nickel hydroxide as its active material, a rare earth element iscontained in the nickel hydroxide as a solid solution.

According to this invention, since the oxygen overvoltage is raised toan appropriate value, a lowering of the charge efficiency at hightemperature can be controlled without lowering a discharge potential.

In this invention, it is preferable to employ the followingconstruction.

(1) At least one of cobalt and zinc is also contained as a solidsolution to the nickel hydroxide.

In this construction, in addition to the foregoing effect offered by therare earth element, such effects are brought about that a chargereaction potential at high temperature becomes improved without loweringa discharge potential, a conductivity in nickel hydroxide particles isimproved, and an utilization is improved, when the rare earth elementand Co are contained in the nickel hydroxide as a solid solution.

In addition to the foregoing effect offered by the rare earth element,crystal structure of the nickel hydroxide are deformed to improve anutilization of the active material and an electrode swelling due toformation of γ-NiOOH is controlled, when the rare earth element and Znare contained in the nickel hydroxide as solid solutions.

The foregoing effects are brought about in a combined manner when therare earth element and Co and Zn are included in solid solution state.

(2) The rare earth element is composed at least one kind of ytterbium,europium, yttrium, holmium, lutetium, thulium, and erbium.

(3) The active material has a component shown by the following equation.(In the equation, X is composed at least one kind of ytterbium,europium, lutetium and erbium; and a=b+c+d, 0.02≦a≦0.20, 0≦c<0.20,0≦d<0.20).

    (Ni.sub.1-a X.sub.b Co.sub.c Zn.sub.d)(OH).sub.2

A fifth invention of this application also relates to a nickelelectrode. The invention is characterized by that, in a nickel electrodeusing a nickel hydroxide as its active material, the active material ismixed with a cobalt compound and simple substance or compound of atleast one kind of rare earth element group comprising yttrium, holmium,erbium, thulium, ytterbium, and lutetium.

In this invention, an oxygen evolution potential at the end of chargeremarkably shifts to a noble side when Y, Ho, Er, Tm, Yb and Lu areadded to the nickel hydroxide. FIG. 1 shows the η value of the nickelelectrode at 20° C. when oxide powder of rare earth element is added inequal mole quantity. Incidentally, the η value means a potentialdifference between the oxidation potential and the oxygen evolutionpotential. For example, when the nickel hydroxide is charged at a hightemperature higher than 40° C., the η value becomes small to lower thecharge efficiency. However, when Y, Ho, Er, Tm, Yb and Lu etc. areadded, the η value becomes large effectively to control occurrence ofcompetitive reaction and improve the charge efficiency.

While, the utilization of active material is improved because aconductive network composed mainly of cobalt oxyhydroxide is formed onsurfaces of nickel hydroxide particles, pore insides and surfaces ofelectrode substrate etc., by means of the added cobalt compound. In thiscase, the cobalt oxyhydroxide is obtained by oxidizing cobalt monoxide,α cobalt hydroxide, β cobalt hydroxide and metallic cobalt etc. inalkaline solution.

In this invention, it is preferable to employ the followingconstructions.

(1) At least ytterbium and lutetium were selected from the rare earthelement group.

Especially, Yb and Lu offer foregoing effects conspicuously.

(2) Two or more kinds of elements were selected from the rare earthelement group, and used under mixed state or as a composite compound.

When these elements are used under mixed state, the most appropriateeffect can be brought about. A kind of composite compound, or acomposite compound having Yb and Lu as its principal component, forexample, is inexpensive because it is formed as an eutectoid whenseparating and forming the rare earth element from ore.

(3) In the two or more kinds of selected rare earth elements, contentsof ytterbium and lutetium are larger than or equal to 35 wt % whenconverted to an amount of oxide, and a ratio of the content of yttribiumto the contents of ytterbium and lutetium is larger than or equal to0.75 when converted to an amount of oxide.

(4) The cobalt compound is composed at least one kind of cobaltoxyhydroxide, cobalt monoxide, α cobalt hydroxide, β cobalt hydroxideand metallic cobalt.

(5) In the selected cobalt compound, a percentage of metallic cobalt issmaller than or equal to 3 wt %.

When the metallic cobalt is added, a thickness of conductive layercomprising the cobalt oxyhydroxide increases electrochemically and aconductivity of not-reacting metallic cobalt is added. For this reason,a high-rate discharge characteristic is further improved. However, inorder to reduce a charge reserve quantity in a sealed-type battery andto control an increase in the cost, it is desirable to set the amount ofmetallic cobalt to a value smaller than or equal to 3 wt % in relationto the total amount of cobalt compound.

A sixth invention of this application relates to an alkaline storagebattery. This invention is characterized by that, in an alkaline storagebattery equipped with a nickel electrode using a nickel hydroxide forits active material, a negative electrode, a separator and an alkalielectrolyte; simple substance or compound of rare earth element iscoated on a surface of the nickel electrode.

In this invention, simple substance or compound of Yb, for example, isdeposited as a stable hydroxide in the alkali aqueous solution. Thishydroxide has an effect to raise the oxygen overvoltage at hightemperature, so that the evolution of oxygen from the positive electrodeside at final stage of charging is controlled and the utilization athigh temperature is improved. Further, the rare earth element isslightly eluded into the electrolyte and deposited on a surface of thehydrogen absorbing alloy as a stable hydroxide to form a protectivecoat. For this reason, dissolution of the rare earth element in thehydrogen absorbing alloy is controlled, so that the alloy is preventedfrom being corroded and the battery service life is prolonged.

By the way, the dissolution control effect owing to the rare earthelement such as Yb, etc. is also brought about on the cobalt compound.Therefore, formation of HCoO₂ ⁻ ion due to the dissolution of cobaltcompound is controlled. In such a case, the conductive network betweenactive materials owing to CoOOH formed by the first cycle charging isformed insufficiently, so that the utilization is reduced and thehigh-rate discharge characteristic performance is impaired. However,since the rare earth element is coated on the surface of nickelelectrode in this invention, the rare earth element is located apartfrom the cobalt compound. Accordingly, there is no chance for the cobaltcompound in the electrode to be controlled in its dissolution, so thatthe conductive network is formed securely.

A seventh invention of this application also relates to an alkalinestorage battery. This invention is characterized by that, in an alkalinestorage battery equipped with a nickel electrode using a nickelhydroxide as its active material, a negative electrode, a separator andan alkali electrolyte; simple substance or compound of rare earthelement is coated on a surface of the separator.

Also in this invention, the utilization at high temperature is improved,the hydrogen absorbing alloy is prevented from being corroded, and theconductive network is formed securely, in the same way as the foregoingsixth invention. In addition, since a net charge quantity of the nickelhydroxide in the nickel electrode does not decrease in this invention,there is no chance for the battery capacity or energy density todecrease.

In this invention, the rare earth element is coated at least on apositive electrode side face of the separator. Thereby, the oxygenovervoltage of the nickel hydroxide is raised positively and theutilization at high temperature is improved securely.

In the foregoing sixth and seventh inventions, a weight percentage ofcoating is preferably set to 0.1 wt %˜10 wt % of an amount of positiveelectrode active material. The effect to raise the oxygen overvoltagecan not be obtained when the percentage is smaller than 0.1 wt %. Whenthe percentage is larger than 10 wt %, the dissolution control effectbecomes too large, so that disadvantages will arise such as a faultyformation of the conductive network and a delay of activation of thehydrogen absorbing alloy of the negative electrode.

An eighth invention of this application also relates to an alkalinestorage battery. This invention is characterized by that, in an alkalinestorage battery equipped with a nickel electrode using a nickelhydroxide as its active material, a negative electrode, a separator andan alkali electrolyte; simple substance or compound of rare earthelement is dissolved in the alkali electrolyte.

Also in this invention, the utilization at high temperature is improved,the hydrogen absorbing alloy is prevented from being corroded, and theconductive network is formed securely, in the same way as the foregoingsixth invention.

In this invention, it is desirable that the alkali electrolyte has aprincipal component of potassium hydroxide and contains sodium hydroxideor lithium hydroxide. Thereby, the high temperature characteristic isimproved. Conventionally, in case of the sodium hydroxide contained,there has been such a problem that its content has been restricted inorder to prevent the electrolyte from becoming viscous. In case of thelithium hydroxide contained, there has been such a problem that alithium ion concentration in the electrolyte has been reduced with aprogress of the charge/discharge cycle. In this invention, however, suchproblems can be solved because the rare earth elements are dissolved inthe electrolyte.

In the above-mentioned sixth, seventh and eighth inventions, it ispreferable to employ the following constructions.

(1) The rare earth element is composed of ytterbium.

(2) The compound of rare earth element is composed of oxide, hydroxideor halogenide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the η value at 20° C. of the nickel electrodewhen the oxide powders of rare earth elements are added in the same molequantity in relation to the fifth invention of this application.

FIG. 2 is a graph showing relations of the discharge capacity and thebattery internal pressure with the cycle for batteries of example 1.

FIG. 3 is a graph showing the charge/discharge cycle characteristic forbatteries of example 2.

FIG. 4 is a graph showing relations of the discharge capacity and thebattery internal pressure with the cycle for batteries of example 3.

FIG. 5 is a graph showing relations between the cycle number and thedischarge capacity for electrodes of example 4.

FIG. 6 is a graph showing relations of the discharge capacity and thebattery internal pressure with the cycle for batteries of example 4.

FIG. 7 is a graph showing relations between the cycle number and thedischarge utilization for batteries of example 5.

FIG. 8 through FIG. 10 are graphs showing results of X-ray diffractionmeasurements for hydrogen absorbing alloys of negative electrodes ofexample 5.

FIG. 11 is a graph showing relations between the Yb₂ O₃ addition amountand the discharge utilization in example 6.

FIG. 12 is a graph showing relations between the temperature change andthe positive electrode capacity utilization for electrodes of example 7.

FIG. 13 is a graph showing relations between the temperature change andthe battery capacity for batteries of example 8.

FIG. 14 is a graph showing relations between the temperature change andthe positive electrode capacity utilization for electrodes of example 9.

FIG. 15 is a graph showing relations between the high-temperature chargeefficiency and the Yb content for batteries of example 11.

FIG. 16 is a graph showing relations between the high-temperature chargeefficiency and the Co content for batteries of example 11.

FIG. 17 is a graph showing relations between the high-temperature chargeefficiency and the Zn content for batteries of example 11.

FIG. 18 is a graph showing relations between the cycle number and theutilization for electrodes of example 13.

FIG. 19 is a graph showing the charge curves at 20° C. and 50° C. forelectrodes of example 13.

FIG. 20 is a graph showing the first cycle charge curve for electrodesof example 13.

FIG. 21 is a graph showing the high-rate discharge characteristic forelectrodes of example 13.

FIG. 22 is a graph showing results of the temperature characteristictest for batteries of example 14.

FIG. 23 through FIG. 28 are graphs showing the X-ray diffractionpatterns for active materials of nickel electrodes of example 14.

FIG. 29 through FIG. 33 are graphs showing the X-ray diffractionpatterns for active materials of hydrogen absorbing electrodes ofexample 14.

FIG. 34 is a graph showing relations between the temperature and theutilization for batteries of example 15.

FIG. 35 is a graph showing the charge curves at 20° C. and 40° C. forbatteries of example 15.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLE 1

This example relates to a hydrogen absorbing electrode.

[Making-up of Electrode]

A commercially available Mm (misch metal), which is a composite of rareearth elements of La, Ce, Pr & Nd etc., and Ni, Al, Co & Mn were weighedfor specified quantities. An alloy having a component of MmNi₃.8 Al₀.3Co₀.7 Mn₀.2 was made up using a high-frequency melting furnace under aninert atmosphere. This is named as an alloy X.

The alloy X was mechanically ground, and the resulting powder wassufficiently mixed with 0.5 wt % of CeO₂ powder using a mortar and addedwith a thickener so as to be formed into a paste. This paste was filledin a nickel fiber substrate, dries and pressed so as to make up ahydrogen absorbing electrode. This is named as an example electrode 1A.

On the other hand, example electrodes 1B, 1C & 1D were made up in thesame procedures as those of the example electrode 1A except that Gd₂ O₃,Er₂ O₃ and Yb₂ O₃ were used in place of the CeO₂.

Further, a comparison electrode 1a was made up without mixing the rareearth element such as CeO₂, that is; by using the alloy X only, in thesame procedures as those of the example electrode 1A.

[Making-up of Battery]

Utilizing the foregoing electrodes for serving as negative electrodes,paste-type nickel electrodes, which used high-density nickel hydroxidepowder as active material, for serving as positive electrodes, andpotassium hydroxide aqueous solution having a specific gravity of 1.28for serving as an electrolyte; sealed nickel hydride storage batteriesof AA-size with nominal capacity of 1,100 mAh were made up. Thebatteries thus made up were named as example batteries 1A, 1B, 1C & 1Dand a comparison battery 1a in correspondence with the exampleelectrodes 1A, 1B, 1C & 1D and the comparison electrode 1a.

[Test]

Charge/discharge cycle tests were done on the foregoing batteries. Thecharging was done for three hours using a current of 0.5 CmA, and thedischarging was done down to 1.0 V using a current of 0.5 CmA. A restingtime between the charging and discharging was one hour.

FIG. 2 is the graph showing relations of the discharge capacity and thebattery internal pressure with the cycle. As obvious from FIG. 2, theexample batteries 1A, 1B, 1C & 1D are excellent in the charge/dischargecharacteristic as compared with the comparison battery 1a. Further,these batteries were remarkably improved in terms of the batteryinternal pressure, too. Especially, the example batteries 1C & 1D wereexcellent in this respect. The example battery 1A was speciallyexcellent in the battery internal pressure characteristic and batteryvoltage characteristic.

The foregoing batteries were disassembled, the hydrogen absorbing alloyswere taken out of the electrodes after being subjected to thecharge/discharge cycles, and the X-ray diffractions of them weremeasured. Comparing peaks of rare earth element hydroxides in themeasured results, it was found that the example electrodes 1A, 1B, 1C &1D produced small quantities of formed hydroxides and controlled interms of occurrence of alloy corrosion as compared with the comparisonelectrode 1a. As the results, the decrease in charge reserve quantitycaused by alloy corrosion is avoided and the battery internal pressurerise due to produced hydrogen is controlled.

As mentioned above, according to the hydrogen absorbing electrode ofthis example, the battery internal pressure rise can be controlled andthe excellent charge/discharge cycle characteristic can be broughtabout.

The rare earth elements such as Ce etc. are used for serving as theoxides in the foregoing example, however, these elements may be used ashydroxides and halogenides. Even in this case, the same effect will bebrought about.

It is permitted for the rare earth elements to exist in grain boundariesof the hydrogen absorbing alloy particles and on surfaces of the alloyparticles.

EXAMPLE 2

This example relates to a hydrogen absorbing electrode.

[Making-up of Electrode]

An hydrogen absorbing alloy having a component of MmNi₃.6 Al₀.3 Co₀.75Mn₀.35 was ground, by using a ball mill, to make powder having a grainsize of 50 μm. The powder was sufficiently mixed with 0.5 wt % of Eroxide, added with 0.5 wt % of methyl cellulose aqueous solution as athickener, and adjusted its viscosity so as to be formed into paste-likeliquid. This viscous liquid was filled in a fibrous nickel poroussubstrate, dried and pressed to a specified thickness. Thus, a hydrogenabsorbing electrode was made up. This is named as an example electrode2A.

An example electrode 2B was made up in the same procedures as those ofthe example 2A except that Yb oxide was used in place of the Er oxide.

On the other hand, a comparison electrode 2a was made up in the sameprocedures as those of the example electrode 2A except that the rareearth element oxide was not mixed.

[Making-up of Battery]

Utilizing the foregoing electrodes for serving as negative electrodes,nickel electrodes having larger capacities that those for serving aspositive electrodes, separators comprising polypropylene non-wovencloths, and alkali aqueous solution; nickel hydride storage batteriescontrolled in terms of the negative electrode capacities, were made up.These batteries thus made up were named as example batteries 2A & 2B anda comparison battery 2a in correspondence with the example electrodes 2A& 2B and the comparison electrode 2a.

[Test]

Charge/discharge cycle characteristics were examined on the foregoingbatteries. The charging was done for five hours using a current of 0.3C, and the discharging was done down to 1.0V using a currentcorresponding to 1C, repeatedly. Thus, elapses of negative electrodecapacities were checked.

FIG. 3 is the graph showing the elapse of capacity on every cycle withreference to the first cycle capacity as 100. As obvious from FIG. 3,the example batteries 2A & 2B were remarkably improved in terms of thecharge/discharge cycle characteristics.

As mentioned above, according to the hydrogen absorbing electrode ofthis example, the battery internal pressure rise can be controlled andthe excellent charge/discharge cycle characteristic can be broughtabout.

The rare earth elements such as Er etc. are used for the oxides in theforegoing example, however, these elements may be used for thehydroxides and halogenides. Even in this case, the same effect will bebrought about.

EXAMPLE 3

This example relates to a hydrogen absorbing electrode.

[Making-up of Electrode]

An alloy X having a component of MmNi₃.8 Al₀.3 Co₀.7 Mn₀.2 was made upin the same procedures as those of the example 1.

On the other hand, an alloy 3A was made up in the same procedures asthose of the alloy X except that Sm was added to the component of thealloy X so as to obtain a weight ratio of Mm to Sm as 95:5. Thepercentages of Ni, Al, Co and Mn were the same as those of the alloy X.

In the same way, an alloy 3B was made up in the same procedures as thoseof the alloy 3A except that Gd was used in place of Sm.

In the same way, an alloy 3C was made up in the same procedures as thoseof the alloy 3A except that Er was used in place of Sm.

In the same way, an alloy 3D was made up in the same procedures as thoseof the alloy 3A except that Yb was used in place of Sm.

The foregoing alloys were mechanically ground, and the resulting alloypowders were added with a thickener so as to be formed into pastes.These pastes were filled in nickel fiber substrates, dried and pressedso as to make up hydrogen absorbing electrodes. The electrodes thus madeup were named as example electrodes 3A, 3B, 3C & 3D and an comparisonelectrode 3a in correspondence with the alloys 3A, 3B, 3C & 3D and thealloy X.

[Making-up of Battery]

Utilizing the foregoing electrodes, batteries were made up in the sameprocedures as those of the example 1. These batteries thus made up werenamed as example batteries 3A, 3B, 3C & 3D and a comparison battery 3ain correspondence with the example electrodes 3A, 3B, 3C & 3D and thecomparison electrode 3a.

[Test]

Tests same as those of the example 1 were done.

FIG. 4 is the graph showing relations of the discharge capacity and thebattery internal pressure with the cycle. As obvious from FIG. 4, theexample batteries 3A, 3B, 3C & 3D are excellent in terms of thecharge/discharge characteristic as compared with the comparison battery3a. Further, these batteries were remarkably improved in terms of thebattery internal pressure, too.

The foregoing batteries were disassembled, the hydrogen absorbing alloyswere taken out of the electrodes after being subjected to thecharge/discharge cycles, and the X-ray diffractions of them weremeasured. Comparing peaks of rare earth element hydroxides in themeasured results, it was found that the example electrodes 3A, 3B, 3C &3D produced small quantities of formed hydroxides and controlled interms of occurrence of alloy corrosion, as compared with the comparisonelectrode 3a.

As mentioned above, according to the hydrogen absorbing electrodes ofthis example, the battery internal pressure rise can be controlled andthe excellent charge/discharge cycle characteristic can be broughtabout.

EXAMPLE 4

This example relates to a hydrogen absorbing electrode.

[Making-up of Electrode]

The alloy 3D made up in the example 3 was mechanically ground to obtainalloy powder, and this alloy powder was dipped and stirred inhigh-temperature alkali aqueous solution formed by mixing KOH with LiOH.Then, it was rinsed and dried. In this instance, the alkali aqueoussolution was identical with that used for an electrolyte. Thereafter,the alloy powder after being dried was added with a thickener to beformed into a paste. It was then filled in a nickel fiber substrate,dried and pressed so that a hydrogen absorbing electrode was made up.This is named as an example electrode 4A.

An example electrode 4B was made up in the same procedures as those ofthe example electrode 4A except that acetic-acid/sodium-acetate buffersolution adjusted its pH to 3.6 was used in place of thehigh-temperature alkali aqueous solution.

A comparison electrode 4a was made up in the same procedures as those ofthe example electrode 4A except that the dipping treatment was not done.

[Test of Electrode]

Utilizing the above-mentioned electrodes, charging and discharging werecarried out by using ordinary nickel electrodes as their counterelectrodes. FIG. 5 shows these results. As obvious from FIG. 5, initialactivations of the example electrodes 4A and 4B were quickly done,especially that of the example electrode 4B was quickly done, so thattheir capacities were large.

[Making-up of Battery]

Utilizing the foregoing electrodes, batteries were made up in the sameprocedures as those of the example 1. These batteries thus made up werenamed as example batteries 4A & 4B and a comparison battery 4a incorrespondence with the example electrodes 4A & 4B and the comparisonelectrode 4a.

[Test]

Tests same as those of the example 1 were done.

FIG. 6 is the graph showing relations of the discharge capacity and thebattery internal pressure with the cycle. As obvious from FIG. 6, theexample batteries 4A & 4B are excellent in terms of characteristics ofthe discharge capacity of the charge/discharge cycle and the batteryinternal pressure, as compared with the comparison battery 4a. In FIG.6, the discharge capacities are plotted at upper side and the batteryinternal pressures are plotted at lower side.

As mentioned above, according to the hydrogen absorbing electrodes ofthis example, the battery internal pressure can be controlled and theexcellent charge/discharge cycle characteristic can be obtained.Further, excellent capacity characteristic can be obtained from aninitial stage.

EXAMPLE 5

This example relates to a nickel electrode.

[Making-up of Electrode]

Commercially available nickel hydroxide powder not containing cadmiumwas added with 6 wt % of metallic Co powder and 4 wt % of CoO powder forserving as a conductive agent, and further added with 2.5 wt % of Yb₂ O₃powder. Then, they were sufficiently mixed and added with water and athickener to be formed into a paste. The paste was filled in a nickelfiber substrate, dried and pressed, so as to make up a nickel electrode.This is named as an example electrode 5A.

A comparison electrode 5a was made up in the same procedures as those ofthe example electrode 5A except that Ca(OH)₂ was added in place of Yb₂O₃.

Further, a comparison electrode 5b was made up in the same procedures asthose of the example electrode 5A except that no compound was added inplace of Yb₂ O₃.

[Making-up of Battery]

Utilizing the foregoing electrodes for serving as positive electrodesand ordinary hydrogen absorbing electrodes for serving as negativeelectrodes, and using an alkali electrolyte; nickel hydride storagebatteries were made up. The batteries thus made up are named as anexample battery 5A and comparison batteries 5a & 5b in correspondencewith the example electrode 5A and the comparison electrodes 5a & 5b.

[Test]

Charge/discharge cycle tests were done on the foregoing batteries. FIG.7 shows these results. Table 1 shows the potential difference (η value)between the oxygen evolution potential and oxidation potential ofbattery.

    [TABLE 1]                                                                     ______________________________________                                                battery                                                                             η value                                                     ______________________________________                                                5A    60 mV                                                             5a 53 mV                                                                      5b 51 mV                                                                    ______________________________________                                    

The discharge utilization of FIG. 7 is calculated as follows. Apractical discharge capacity at time when a theoretical capacity ofNi(OH)₂ in a positive electrode composite is assumed as 290 mAh per onegram, is divided by the theoretical capacity and multiplied by 100. Asobvious from FIG. 7, the example battery 5A maintains a sufficientcapacity even at a high temperature. Further, as obvious from Table 1,the η value of the example battery 5A is larger than those of thecomparison batteries 5a & 5b. Thereby, decomposition of the electrolyteis controlled to prevent the battery capacity from being reduced.

Further, the batteries after being subjected to the charge/dischargecycle tests were disassembled, the hydrogen absorbing alloys were takenout of the electrodes, and X-ray diffractions of them were measured.Results of them are shown in FIG. 8 through FIG. 10. As obvious fromthese figures, in the example battery 5A, peaks of the rare earthelement hydroxides in the vicinity of 2θ=27°˜29° are small so that thecorrosion of alloy is controlled.

As mentioned above, according to the nickel electrodes of this example,the utilization of nickel electrode at high temperature can be increasesand a service life of the hydrogen absorbing electrode can be prolongedwhen used for the nickel hydride storage battery.

EXAMPLE 6

This example relates to a nickel electrode.

[Making-up of Electrode and Battery]

Batteries were made up in the same procedures as those of the example 5except that the addition amount of Yb₂ O₃ was set variously. Theaddition amount (wt %) of Yb₂ O₃ was varied as 0, 0.1, 0.5, 1.0, 2.5,5.0 and 10.0.

[Test]

Discharge utilizations at various temperature were measured on theforegoing batteries. FIG. 11 shows these results. As obvious from FIG.11, high-temperature performances are improved by adding Yb₂ O₃.However, the utilization at ordinary temperature becomes worse when theaddition amount is larger than 5 wt % and the utilization at hightemperature becomes worse when the addition amount is smaller than 0.5wt %. Therefore, it is desirable to set the addition amount within arange of 0.5 to 5.0 wt %.

EXAMPLE 7

This example relates to a nickel electrode.

[Making-up of Electrode]

High-density spherical nickel hydroxide powder containing 5 wt % of Znas a solid solution was added to 10 wt % of CoO powder. This is named asa mixed powder X1.

The mixed powder X1 was mixed with a thickener to be formed into apaste. The paste was filled in a nickel porous substrate to make up anickel electrode. This is named as a comparison electrode 7a.

On the other hand, the mixed powder X1 was sufficiently mixed with 2.5wt % of holmium oxide powder, and added with a thickener to be formedinto a paste. The paste was filled in a nickel porous substrate to makeup a nickel electrode. This is named as an example electrode 7A.

Example electrodes 7B & 7C were made up in the same procedures as thoseof the example electrode 7A except that erbium oxide powder andytterbium oxide powder were added respectively, in place of the holmiumoxide powder.

Comparison electrodes 7b, 7c & 7d were made up in the same procedures asthose of the example electrode 7A except that lanthanum oxide powder,cerium oxide powder and gadolinium oxide powder were added respectively,in place of the holmium oxide powder.

[Making-up of Battery]

The foregoing electrodes were wrapped by nylon separators and ordinaryhydrogen absorbing electrodes were utilized for negative electrodes, sothat nickel hydride storage batteries were made up. The batteries thusmade up are named as example batteries 7A, 7B & 7C and comparisonbatteries 7a, 7b, 7c & 7d in correspondence with the example electrodes7A, 7B & 7C and comparison electrodes 7a, 7b, 7c & 7d.

[Test]

Making the positive electrode capacity smaller than the negativeelectrode capacity, charge/discharge cycle tests were done in potassiumhydroxide aqueous solution having a specific gravity of 1.28. Thecharging was carried out for 15 hours using a current of 30 mA(corresponding to 0.1 C), and the discharging was terminated at 0Vrelative to Hg/HgO reference electrode using a current of 60 mA.

FIG. 12 shows the relations between the temperature change and thepositive electrode capacity utilization (percentage relative totheoretical capacity of positive electrode). As obvious from FIG. 12,the utilizations lower extremely with a rise of temperature in thecomparison electrodes 7a, 7b, 7c & 7d, however, an extent of lowering issmall in the example electrodes 7A, 7B & 7C. Especially, the extent oflowering is very small in the example electrode 7C, so that thiselectrode maintains a stable capacity even at a low temperature.

As mentioned above, according to the nickel electrodes of this example,a range of increase and decrease in capacity can be made small over awide temperature region from low to high temperatures. Thus, a stabilitycan be improved.

EXAMPLE 8

This example relates to a nickel electrode.

[Making-up of Electrode]

High-density spherical nickel hydroxide powder containing 3 wt % of Znas a solid solution was added to 10 wt % of CoO powder. This is named asa mixed powder X2.

The mixed powder X2 was mixed with a thickener to be formed into apaste. The paste was filled in a nickel porous substrate to make up anickel electrode. This is named as a comparison electrode 8a.

On the other hand, the mixed powder X2 was sufficiently mixed with 2.5wt % of ytterbium oxide powder, and added with a thickener to be formedinto a paste. The paste was filled in a nickel porous substrate to makeup a nickel electrode. This is named as an example electrode 8A.

High-density spherical nickel hydroxide powder containing 3 wt % of Znand 3 wt % of Co as a solid solution was mixed to 10 wt % of CoO. Themixed powder was sufficiently mixed with 2.5 wt % of ytterbium oxidepowder, and added with a thickener to be formed into a paste. The pastewas filled in a nickel porous substrate to make up a nickel electrode.This is named as an example electrode 8B.

High-density spherical nickel hydroxide powder containing 3 wt % of Znand 5 wt % of Co as a solid solution was mixed to 10 wt % of CoO. Themixed powder was sufficiently mixed with 2.5 wt % of ytterbium oxidepowder, and added with a thickener to be formed into a paste. The pastewas filled in a nickel porous substrate to make up a nickel electrode.This is named as an example electrode 8C.

[Making-up of Battery]

Utilizing the foregoing electrodes, AA-size nickel hydroxide batterieswith capacity of 1,100 mAh were made up according to a well-knownmethod. The batteries thus made up were named as example batteries 8A,8B & 8C and a comparison battery 8a in correspondence with the exampleelectrodes 8A, 8B & 8C and the comparison electrode 8a.

[Test]

Charge/discharge cycle tests were done. The charging was carried out for15 hours using a current of 100 mA, and the discharging was terminatedat 1.0V using a current of 200 mA.

FIG. 13 shows the relations between the temperature change and thebattery capacity. As obvious from FIG. 13, the capacity reductions dueto the temperature change are small in the example batteries 8A, 8B & 8Cas compared with the comparison battery 8a. The capacity of thecomparison battery 8a at a high temperature higher than 40° C. lowers toa value smaller than 50% of that at 20° C. However, especially thecapacity of the example battery 8C even at a temperature as high as 60°C. keeps a value equal to 70% of that at 20° C. A difference betweencapacities of the example battery 8A and the example battery 8Bindicates that a charge efficiency becomes larger by synergetic effectoffered between Co as a solid solution and rare earth element in theexample battery 8B.

As mentioned above, according to the nickel electrode of this example, arange of increase and decrease in capacity can be made small over a widetemperature region from low to high temperatures. Thus, a stability canbe improved.

EXAMPLE 9

This example relates to a nickel electrode.

[Making-up of Eelectrode]

High-density spherical nickel hydroxide powder containing 5 wt % of Znas a solid solution was added to 10 wt % of CoO powder. This is named asa mixed powder X1.

The mixed powder X1 was mixed with a thickener to be formed into apaste. The paste was filled in a nickel porous substrate to make up anickel electrode. This is named as a comparison electrode 9a.

On the other hand, the mixed powder X1 was sufficiently mixed with 2.5wt % of ytterbium oxide powder, and added with a thickener to be formedinto a paste. The paste was filled in a nickel porous substrate to makeup an example electrode 9A.

Commercially available ytterbium nitrate solution was neutralized byalkali to prepare a hydroxide. An example electrode 9B was made up inthe same procedures as those of the example electrode 9A except that 2.5wt % of the above hydroxide powder was sufficiently mixed to the mixedpowder X1.

An example electrode 9C was made up in the same procedures as those ofthe example electrode 9A except that the mixed powder X1 wassufficiently mixed to 2.5 wt % of commercially available ytterbiumfluoride.

[Making-up of Battery]

Utilizing the foregoing electrodes, nickel hydride storage batterieswere made up in the same procedures as those of the example 7. Thebatteries thus made up were named as example batteries 9A, 9B & 9C and acomparison battery 9a in correspondence with the example electrodes 9A,9B & 9C and the comparison electrode 9a.

[Test]

Charge/discharge cycle tests same as those of the example 7 were done.

FIG. 14 shows the relations between the temperature change and thepositive electrode capacity utilization (percentage relative totheoretical capacity of positive electrode). As obvious from FIG. 14,the utilizations extremely lower with a rise of temperature in thecomparison electrode 9a, however, high capacities are offered by theexample electrodes 9A, 9B & 9C even at 50° C.

As mentioned above, according to the nickel electrode of this example, arange of increase and decrease in capacity can be made small over a widetemperature region from low to high temperatures. Thus, a stability canbe improved.

EXAMPLE 10

This example relates to a nickel electrode.

[Making-up of Electrode]

High-density spherical nickel hydroxide powder containing 5 wt % of Znas a solid solution and having an internal pore volume of 0.03 ml/g wasmixed with 10 wt % of CoO powder. The mixed powder was sufficientlymixed with 2.5 wt % of ytterbium oxide powder, and added with athickener to be formed into a paste. The paste was filled in a nickelporous substrate to make up a nickel electrode. This is named as anexample electrode 10A.

On the other hand, the nickel hydroxide powder prepared by aconventional neutralizing method and containing 5 wt % of Zn as a solidsolution and having an internal pore volume of 0.14 ml/g, wassufficiently mixed with 2.5 wt % of ytterbium oxide powder, and addedwith a thickener to be formed into a paste. The paste was filled in anickel porous substrate to make up a nickel electrode. This is named asa comparison electrode 10a.

[Making-up of Battery]

Utilizing the foregoing electrodes, nickel hydride storage batterieswere made up in the same procedures as those of the example 7. Thebatteries thus made up were named as an example battery 10A and acomparison battery 10a in correspondence with the example electrode 10Aand the comparison electrode 10a.

[Test]

Charge/discharge cycle tests same as those of the example 7 were done.

The charge/discharge cycle tests carried out at 20° C. proved that theutilization of positive electrode capacity was 100% for the exampleelectrode 10A but it was 96% for the comparison electrode 10a. Thecharge/discharge cycle tests carried out at 50° C. proved that theutilization of positive electrode capacity was 72% for the exampleelectrode 10A but it was 61% for the comparison electrode 10a. It wasfound, in discharge tests using a current of 1,800 mA (corresponding to3C), that the utilization of the comparison electrode 10a becameextremely small but a high capacity was provided by the exampleelectrode 10A.

As mentioned above, according to the nickel electrodes of this example,a range of increase and decrease in capacity can be made small over awide temperature region from low to high temperatures. Thus, a stabilitycan be improved. In addition, high-rate discharge characteristics can beimproved, the stability at high temperature and the high capacity can beachieved.

In the above description, the addition amount of ytterbium oxide powderis 2.5 wt %, however, an addition amount smaller than this value mayprovide a sufficient utilization at high temperature. An addition amountlarger than 2.5 wt % will further increase the utilization at hightemperature, however, an utilization up to and around 20 wt % ispreferable in consideration of the cost problem.

EXAMPLE 11

This example relates to a nickel electrode.

[Making-up of Nickel Hydroxide Powder and Electrode]

Nickel electrodes having components as listed in Table 2 were made up inthe following procedures.

Sodium hydroxide aqueous solution was dropped in and stirred withaqueous solution prepared by adding a specified quantity of ytterbiumnitrate to nickel nitrate. A pH of it was kept to within a range of 11to 14, and nickel hydroxide particles were deposited, rinsed and dried.Thus, nickel hydroxide powder containing Yb as a solid solution wasprepared. The nickel hydroxide powder was mixed with CoO powder as aconductive adjuvant, and added with aqueous solution dissolving athickener to be formed into a paste. The paste was filled in nickelfiber substrates, dried and pressed to a specified thickness. Thus,nickel electrodes were made up. Thereby, example electrodes 11A & 11Bwere prepared.

Nickel hydroxide powder containing Yb and Co as a solid solution wasprepared in the same procedures as those of the example electrode 11Aexcept that cobalt nitrate was added together with the ytterbiumnitrate. Further, an example electrode 11C was made up in the sameprocedures.

Nickel hydroxide powder containing Yb and Zn as a solid solution wasprepared in the same procedures as those of the example electrode 11Aexcept that zinc nitrate was added together with the ytterbium nitrate.Further, an example electrode 11D was made up in the same procedures.

Nickel hydroxide powder containing Yb, Zn and Co as a solid solution wasprepared in the same procedures as those of the example electrode 11Aexcept that zinc nitrate and cobalt nitrate were added together with theytterbium nitrate. Further, an example electrode 11E was made up in thesame procedures.

Comparison electrodes 11a, 11b & 11c were made up respectively in thesame procedures as those of the example electrodes 11A, 11B & 11C exceptthat the ytterbium nitrate was not added.

    [TABLE 2]                                                                     ______________________________________                                                 Ni(OH).sub.2                                                                             Yb(OH).sub.2                                                                           Co(OH).sub.2                                                                           Zn(OH).sub.2                              electrode wt % wt % wt % wt %                                               ______________________________________                                        11A      97         3        --       --                                        11B 94 5 -- --                                                                11C 92 3 3 --                                                                 11D 94 3 -- 5                                                                 11E 89 3 3 5                                                                  11a 100  -- -- --                                                             11b 95 -- 3 --                                                                11c 97 -- -- 5                                                              ______________________________________                                    

[Making-up of Battery]

Utilizing the foregoing electrodes for serving as positive electrodesand well-known hydrogen absorbing electrodes for serving as negativeelectrodes, electrode groups restricted in their positive electrodecapacities were constructed. Then, potassium hydroxide aqueous solutionhaving a specific gravity of 1.28 was poured excessively for serving asan electrolyte. After leaving them as they were for 24 hours, chargingand discharging were repeated for five cycles to complete fullactivation. One cycle was such that, the charging was done for 15 hoursusing a current corresponding to 0.1 C of the nickel electrodetheoretical capacity, and discharging was done until a potential betweenboth electrodes dropped down to 1V using a current corresponding to 0.2C. Thereby, nickel hydride storage batteries were made up. The batteriesthus made up were named as example batteries 11A, 11B, 11C, 11D & 11Eand comparison batteries 11a, 11b & 11c in correspondence with theexample electrodes 11A, 11B, 11C, 11D & 11E and the comparisonelectrodes 11a, 11b & 11c.

[Test]

Various charge/discharge tests were done on the foregoing batteries.

(1) Relations between high-temperature charge efficiencies and Ybcontents were examined on the example batteries 11A & 11B and thecomparison battery 11a. FIG. 15 shows these results. Test conditionswere such that, under a temperature of 45° C., the charging was done for15 hours using a current corresponding to 0.1 C of the nickel electrodetheoretical capacity, and the discharging was done until a potentialbetween both electrodes got to 1V using a current corresponding to 0.2C. The charge efficiencies at 45° C. were expressed by percentageassuming that a charge efficiency at 20° C. was 100.

As obvious from FIG. 15, the charge efficiency increases with anincrease in the Yb content. This is because an oxygen overvoltage of thenickel hydroxide rises when Yb is contained as a solid solution. As theYb content is increased, the oxygen overvoltage becomes high, apotential difference between the charge reaction and the oxygenevolution reaction can be increased, and the charge efficiency can beimproved.

(2) Relations between high-temperature charge efficiencies and Cocontents were examined on the example batteries 11A & 11C and thecomparison battery 11b. FIG. 16 shows these results. Test conditionswere the same as those of the above (1).

The following facts are understood from FIG. 16. The high-temperaturecharge efficiencies are comparatively large when Co is contained even ifYb is not contained. However, the example batteries 11A & 11C containingYb are superior to the comparison battery 11b in terms of thehigh-temperature charge efficiency. In addition, the example battery 11Ccontaining Co and Yb is superior to the example battery 11A onlycontaining Yb in terms of the high-temperature charge efficiency. Thereason is supposed to be a fact that the potential difference betweencharge reaction and oxygen evolution reaction at high temperature can bemade large by the synergistic effect with Yb because Co has an effect tofurther shift the charge reaction potential at high temperature to thebase side. Moreover, it can be expected that, the conductivity of nickelhydroxide particles is improved and the active material utilization canbe increased, when Co takes a form of higher order oxide. However, it isrequired to limit the Co addition amount to an appropriate range becausethe discharge reaction potential is also shifted to the base side whenCo is added excessively.

(3) Relations between high-temperature charge efficiencies and Zncontents were examined on the example batteries 11A, 11D & 11E and thecomparison battery 11c. FIG. 17 shows these results. Test conditionswere the same as those of the above (1).

The following facts are understood from FIG. 17. The high-temperaturecharge efficiencies are improved in the example battery 11D containingZn & Yb and the example battery 11E containing Zn, Co & Yb. However, thehigh-temperature charge efficiency is rather reduced in the comparisonbattery 11c containing Zn but not containing Yb. Further, the examplebattery 11D containing Zn and Yb is superior to the example battery 11Acontaining Yb only, in terms of the high-temperature charge efficiency.The reason is supposed to be a fact that the potential differencebetween charge reaction and oxygen evolution reaction of the nickelhydroxide can be made large because Zn has an effect to shift the oxygenevolution potential to the noble side. Further, the crystal structure ofnickel hydroxide can be deformed because an ion radius of Zn isdifferent from that of Ni. Therefore, it can be expected that not onlythe active material utilization can be improved but the electrodeswelling due to formation of γ-NiOOH can be controlled. The case whereonly Zn is added is inferior to the case where only Yb is added, interms of the high-temperature charge efficiency in such effect of Zn.Moreover, in case where Zn is added together with Yb or together with Yband Co, the effect of Zn is not impaired, but a synergistic effect withYb and Co can be offered preferably.

As mentioned above, according to the nickel electrode of this example,the charge/discharge efficiency can be improved over a wide range oftemperature, and the stability of capacity characteristic can beimproved.

In this example, the same effect can be obtained when other rare earthelements such as Eu and Er are used in place of Yb.

EXAMPLE 12

This example relates to a nickel electrode.

[Making-up of Electrode]

High-density spherical nickel hydroxide powder containing 5 wt % of Znas a solid solution was added to 10 wt % of CoO powder. This is named asa mixed powder X1.

The mixed powder X1 was mixed with a thickener to be formed into apaste. The paste was filled in a nickel porous substrate to make up anickel electrode. This is named as a comparison electrode 12a.

On the other hand, a composite oxide was formed, in which content ratio(wt % contrast) of Yb₂ O₃ to Lu₂ O₃ was fixed to a specified value. 2.5wt % of this composite oxide were sufficiently mixed to the mixed powderX1 in a mortar, and added with a thickener to be formed into a paste.The paste was filled in nickel porous substrates so as to prepare nickelelectrodes. In this case, the content ratio of Yb₂ O₃ to Lu₂ O₃ wasvaried as 100:0, 85:15, and 75:25. The nickel electrodes thus preparedare named as example electrodes 12A, 12B & 12C.

While, Yb₂ O₃ powder and Lu₂ O₃ powder were mixed at a specified ratio(wt % contrast). 2.5 wt % of the mixed powder was sufficiently mixed tothe mixed powder X1 in a mortar, and added with a thickener to be formedinto a paste. The paste was filled in nickel porous substrates so as toprepare nickel electrodes. In this case, the ratio of Yb₂ O₃ powder toLu₂ O₃ powder was varied as 90:10 and 75:25. The nickel electrodes thusmade up are named as example electrodes 12D & 12E.

An example electrode 12F was made up in the same procedures as those ofthe example electrode 12A except that a composite oxide was formed inwhich a content ratio (wt % contrast) between Ho₂ O₃, Er₂ O₃, Tm₂ O₃,Yb₂ O₃, Lu₂ O₃, & Y₂ O₃ was 15:25:10:30:5:15.

Further, example electrodes 12G, 12H & 12I was made up in the followingprocedures. CoO powder was mixed to metallic Co powder at a specifiedratio (wt % contrast). The ratio was so set that the total cobaltquantities of them were equal each other. 10 wt % (converted to CoO) ofcobalt mixed powder were mixed to high-density spherical nickelhydroxide powder containing 5 wt % of Zn as a solid solution. The nickelmixed powder was mixed with 2.5 wt % of composite oxide of rare earthelements, and added with a thickener to be formed into a paste. Thepaste was filled in nickel porous substrates to make up nickelelectrodes. The composite oxide was such that containing Yb₂ O₃ and Lu₂O₃ at a ratio (wt % contrast) of 85:15. In this case, the mixing ratioof CoO powder and Co metal powder was varied as 9:0.78, 8:1.57 & 7:2.36and the nickel electrodes thus made up are named as example electrodes12G, 12H & 12I.

[Making-up of Battery]

Utilizing the foregoing electrodes, separators and the hydrogenabsorbing electrodes; cells controlled in their positive electrodes wereprepared. Utilizing 6.8-normal sodium hydroxide aqueous solution forserving as an electrolyte, nickel hydride storage batteries were madeup. The batteries thus made up were named as example batteries 12A, 12B,12C, 12D, 12E, 12F, 12G, 12H & 12I and a comparison battery 12a incorrespondence with the example electrodes 12A, 12B, 12C, 12D, 12E, 12F,12G, 12H & 12I and a comparison electrode 12a.

[Test]

Under conditions of sufficient electrolyte, charge/discharge cycle testswere done on the foregoing batteries. The charging was done for 15 hoursusing a current of 0.1 CmA, and the discharging was done until thepositive electrode potential got down to 0V relative to Hg/HgO referenceelectrode using a current of 0.2 CmA.

Table 3 shows active material utilizations of fifth cycle at 20° C.,active material utilizations of fifth cycle at 50° C., and η values(difference between oxidizing potential and oxygen evolution potential)at 20° C. and 40° C., respectively.

    [TABLE 3]                                                                     ______________________________________                                                  20° C.,                                                                         50° C.,                                                utilization utilization 20° C., 40° C.,                        battery factor (%) factor (%) η (mV) η (mV)                         ______________________________________                                        12A       100.3    64.2       65.0  24.8                                        12B 100.3 64.3 66.3 25.2                                                      12C 100.4 65.1 67.2 26.5                                                      12D 100.1 50.2 59.5 11.6                                                      12E 100.6 64.4 66.0 24.9                                                      12F 100.2 65.5 67.4 25.8                                                      12a 100.2 30.4 56.7  2.2                                                    ______________________________________                                    

As obvious from Table 3, the example batteries 12A through 12F offeredlarge η values at high temperature and large utilizations as comparedwith the comparison battery 12a. It was confirmed that the oxygenevolution potentials of the example batteries 12A through 12F wereshifted to the noble side as compared with the comparison battery 12a.

Table 4 shows active material utilizations in 0.2 C discharging and 5 Cdischarging at 20° C.

    [TABLE 4]                                                                     ______________________________________                                                   0.2 C discharging,                                                                         5 C discharging                                          utilization utilization                                                      battery factors (%) factors (%)                                             ______________________________________                                        12B        100.3        84.7                                                    12G 101.4 90.1                                                                12H 102.1 90.6                                                                12I 101.8 92.1                                                                12a 100.2 84.8                                                              ______________________________________                                    

The example batteries 12G, 12H & 12I offer large active materialutilizations in 5 C discharging as compared with the comparison battery12a.

Even when the example electrodes 12A, 12B, 12C, 12D, 12E, 12F, 12G, 12H& 12I are used for the sealed-type battery, rises in battery innerpressure become small because the oxygen evolution reaction in chargingat high temperature is controlled. Therefore, the battery service lifeis prolonged remarkably.

As mentioned above, according to the nickel electrodes of this example,the performance at high temperature can be improved and a high energydensity can be achieved.

The oxides of rare earth element are mixed or oxides are used for thecomposite compound in the above examples, however, the same effect maybe provided when hydroxides or fluorides are used in place of theoxides.

Further, the same effect may be provided when cobalt oxyhydroxide, αcobalt hydroxide or β cobalt hydroxide is used in place of CoO andmetallic Co. However, the quantity of metallic Co is preferably smallerthan or equal to 3 wt % in order to control an increase in the dischargereverse quantity.

EXAMPLE 13

This example relates to an alkaline storage battery.

[Making-up of Nickel Electrode]

High-density nickel hydroxide powder containing Zn and Co as a solidsolution was sufficiently mixed with 10 wt % of CoO powder for servingas a conductive adjuvant, and added with a thickener to be formed into apaste. The paste was filled in a nickel porous substrate, dried andpressed to a specified thickness, so that a nickel electrode was madeup. This is named as a comparison electrode 13a.

A paste prepared by mixing Yb₂ O₃ powder to a thickener was coated on asurface of the foregoing nickel electrode, by a quantity of 2.5 wt %relative to a quantity of the nickel hydroxide, and dried again to makeup an example electrode 13A.

Further, high-density nickel hydroxide powder containing Zn and Co as asolid solution was sufficiently mixed with 10 wt % of CoO powder and 2.5wt % of Yb₂ O₃ powder, and added with a thickener to be formed into apaste. The paste was filled in a nickel porous substrate, dried andpressed to a specified thickness, so that a nickel electrode was madeup. This is named as a comparison electrode 13b.

[Making-up of Battery]

Utilizing the foregoing electrodes for serving as positive electrodes,hydrogen absorbing electrodes for serving as negative electrodes, andpotassium hydroxide aqueous solution having a specific gravity of 1.28for serving as electrolytes; nickel hydride storage batteries were madeup. The batteries thus made up are named as an example battery 13A andcomparison batteries 13a & 13b in correspondence with the exampleelectrode 13A and comparison electrodes 13a & 13b.

[Test]

Charge/discharge cycle tests were done on the foregoing batteries undera state of excessive electrolyte. The charging was done using a currentof 0.1 C and the discharging was done using a current of 0.2 C.

FIG. 18 shows results of the charge/discharge. The example electrode 13Aoffered utilizations larger than those of the comparison electrode 13anot coated with Yb₂ O₃, and differences of them were remarkableparticularly at high temperature of 40° C. and 50° C. The comparisonelectrode 13b containing Yb₂ O₃ offered utilizations equal to those ofthe example electrode 13A.

FIG. 19 shows charge curves for the example electrode 13A and thecomparison electrode 13a at 20° C. and 50° C. Oxygen overvoltages of theboth electrodes 13A and 13a are approximately equal at 20° C. However,in the comparison electrode 13a at 50° C., the oxygen overvoltage doesnot rise even in the final stage of charging. Therefore, it can beunderstood that a charge acceptance is lessened. On the contrary, in theexample electrode 13A, the oxygen overvoltage rises in the final stageof charging. Therefore, it can be understood that the charge acceptanceis not lessened even at 50° C. The comparison electrode 13b offeredutilizations at high temperature same as those of the example electrode13A. This is caused by the oxygen overvoltage rise-up effect owing tothe coated or mixed Yb₂ O₃.

FIG. 20 shows the first cycle charge curves for the example electrode13A and the comparison electrodes 13a & 13b. The charging was done forseven hours using a current of 1/30 C. A part wherein constant potentialcan be seen in a range from 50 mV to 100 mV represents a conductivenetwork formation reaction expressed by the following equation.

    HCoO.sub.2.sup.- →CoOOH+e.sup.-

In the comparison electrode 13b, it can be foreseen that a period ofreaction expressed by the above equation is short and the formation ofconductive network is insufficient because the part of constantpotential is short. This is owing to the fact that the mixed Yb₂ O₃restricts the dissolution of CoO. In the example electrode 13A, however,CoO in the electrode is dissolved smoothly and the conductive network istherefore sufficiently formed because Yb₂ O₃ is coated on the electrodesurface. An extent of formation of the conductive network in the exampleelectrode 13A was approximately equal to that of the comparisonelectrode 13a not containing Yb₂ O₃.

FIG. 21 shows high-rate discharge characteristics of the exampleelectrode 13A and the comparison electrodes 13a & 13b. The comparisonelectrode 13b offers a remarkable drop in the high-rate dischargecharacteristics, as compared with the example electrode 13A and thecomparison electrode 13a. The reason is supposed to the fact that, asdescribed above, the formation of conductive network is insufficient inthe comparison electrode 13b. A large drop in the high-rate dischargecharacteristic is not seen in the example electrode 13A because theformation of conductive network is sufficient in the example electrode13A.

As mentioned above, according to the alkaline storage batteries of thisexample, the utilization of nickel electrode can be made large over awide range of temperature and the drop in utilization at hightemperature can be controlled. The high-rate discharge characteristiccan be improved because the conductive network at initial charging canbe formed sufficiently. Consequently, stable capacity characteristicsand excellent cycle characteristics can be obtained.

EXAMPLE 14

This example relates to an alkaline storage battery.

[Making-up of Battery]

Yb₂ O₃ powder was mixed to aqueous solution dissolving a thickener to beformed into a paste. The paste was coated uniformly on both sides ofcommercially available polyolefin non-woven cloth, and dried to form aseparator. On the other hand, high-density nickel hydroxide powdercontaining Zn and Co as a solid solution was sufficiently mixed with 10wt % of CoO powder for serving as a conductive adjuvant, and added witha thickener to be formed into a paste. The paste was filled in a nickelporous substrate, dried and pressed to a specified thickness. Thus, anickel electrode was made up. An electrode group controlled in itspositive electrode capacity was constructed by the foregoing separator,the nickel electrode and the hydrogen absorbing electrode. Thus, asealed nickel hydride battery was made up by utilizing potassiumhydroxide aqueous solution having a specific gravity of 1.28 for servingas an electrolyte. This is named as an example battery 14A.

An example battery 14B was made up in the same procedures as those ofthe example battery 14A except that Yb₂ O₃ powder was mixed to aqueoussolution dissolving a thickener to be formed into a paste and the pastewas coated on only one side of the commercially available polyolefinnon-woven cloth. In this case, the coated face of separator is solocated as to contact with the nickel electrode.

A comparison battery 14a was made up in the same procedures as those ofthe example battery 14B except that the coated face of separator is solocated as to contact with the hydrogen absorbing electrode.

Further, a comparison battery 14b was made up in the same procedures asthose of the example battery 14A except that nothing was coated on theseparator.

These batteries were sufficiently activated as follows. The electrolytewas poured in these batteries which in turn were left as they were for48 hours at ordinary temperature. Then, charging and discharging wererepeated on them for five cycles to complete full activation. One cyclewas such that, the charging was done for 15 hours using a currentcorresponding to 0.1 C of the nickel electrode theoretical capacity, andthe discharging was done until a potential between both electrodesdropped down to 1V using a current corresponding to 0.2 C.

[Test]

Charge/discharge cycle tests were done on the foregoing batteries toexamine temperature characteristics. Test conditions were as follows.Under several temperature conditions, the charging was done for 15 hoursusing a current corresponding to 0.1 C of the nickel electrodetheoretical capacity, and the discharging was done until a potentialbetween both electrodes dropped down to 1V using a current correspondingto 0.2 C. FIG. 22 shows these results.

As obvious from FIG. 22, the example batteries 14A & 14B and thecomparison battery 14a maintain sufficient capacities even aftersubjected to the charging and discharging at high temperatures, andpresent good capacity recoveries when brought back to ordinarytemperature. This effect is outstanding especially in the examplebatteries 14A & 14B. This is because Yb₂ O₃ is coated on the separator,so that the oxygen overvoltage of nickel hydroxide is raised to makelarge the potential difference between the charge reaction and oxygenevolution reaction, thereby the charge efficiency can be improved.

The foregoing batteries were disassembled after completion of thecharge/discharge tests, active materials after subjected to thedischarging were taken out of the nickel electrodes and the hydrogenabsorbing electrodes. The materials were rinsed and dried, then analyzedby X-ray diffraction.

FIG. 23 through FIG. 28 show X-ray diffraction patterns for the activematerials of the nickel electrodes. FIG. 23 is a general view of theX-ray diffraction pattern for the example battery 14A. FIG. 24, FIG. 25,FIG. 26 and FIG. 27 are partially enlarged views of the X-raydiffraction patterns for the example batteries 14A & 14B and thecomparison batteries 14a & 14b, respectively. Enlarged ranges are thosecorresponding to a part X of FIG. 23. FIG. 28 shows the X-raydiffraction pattern of the active material for the nickel electrodebefore subjected to the activation and charging/discharging.

As obvious from these figures, β-Ni(OH)₂ peaks are conspicuous but CoOpeaks are hardly seen in all the batteries. This fact implies that, inany example batteries 14A & 14B and the comparison battery 14a, thedissolution control effect of Yb₂ O₃ does hardly interfere with thedissolution and deposition of CoO and the conductive network issufficiently formed.

FIG. 29 through FIG. 33 show X-ray diffraction patterns for the activematerials of the hydrogen absorbing electrodes. FIG. 29, FIG. 30, FIG.31 and FIG. 32 are partially enlarged views of the X-ray diffractionpatterns for the example batteries 14A & 14B and the comparisonbatteries 14a & 14b, respectively. FIG. 33 shows the partially enlargedview of X-ray diffraction pattern of the active material for thehydrogen absorbing electrode before subjected to the activation andcharging/discharging.

As obvious from these figures, peaks of rare earth element hydroxidecaused by the alloy corrosion appear in the vicinity of 2θ=27°˜29° inthe comparison battery 14b. In any of the example batteries 14A & 14Band the comparison battery 14a, however, the subject peaks are small sothat the alloy corrosion are controlled.

As mentioned above, according to the alkaline storage batteries of thisexample, the decrease in charge efficiency at high temperature can becontrolled without lowering the discharge potential, electrode capacityand energy density etc. of the nickel electrode. Therefore, thecharge/discharge efficiency can be improved over a wide range oftemperature. Further, the cycle life can be prolonged because the alloycorrosion of the hydrogen absorbing electrode can be controlled.

In this case, rare earth elements such as Er may be used in place of Yb,and a hydroxide such as Yb(OH)₃ or fluoride may be used in place of theoxide such as Yb₂ O₃.

The same effect may be brought about, not only in the nickel hydridestorage battery but in the alkaline storage battery, in which paste-typenickel electrodes having principal component of nickel hydroxide areused for their positive electrodes, such as nickel-cadmium storagebattery and the nickel-zinc storage battery etc.

EXAMPLE 15

This example relates to an alkaline storage battery.

[Making-up of Battery]

High-density nickel hydroxide powder was sufficiently mixed with 10 wt %of CoO powder, and added with a thickener to be formed into a paste. Thepaste was filled in a nickel porous substrate, dried and pressed to aspecified thickness so as to make up a nickel electrode. An electrodegroup controlled in its positive electrode capacity was constructed bythis nickel electrode together with a well-known hydrogen absorbingelectrode and a separator having alkali resistance. Electrolytes havingvarious components as listed in Table 5 were poured into that electrodegroup so as to make up nickel hydride storage batteries. As listed inTable 5, batteries using electrolytes containing Yb are named as examplebatteries 15A & 15B, and those using electrolytes not containing Yb arenamed as comparison batteries 15a & 15b.

    [TABLE 5]                                                                     ______________________________________                                                electrolyte components                                                          KOH     LiOH      NaOH  Yb                                             concen- concen- concen- concen-                                              battery tration tration tration tration                                     ______________________________________                                        15A       6 N     --        --    3 × 10.sup.-3 N                         15B 5 N 1 N 1 N 3 × 10.sup.-3 N                                         15a 6 N -- -- --                                                              15b 5 N 1 N 1 N                                                             ______________________________________                                    

[Test]

Charge/discharge cycle tests were done on the foregoing batteries usingHg/HgO electrodes for serving as reference electrodes under open andexcessive electrolyte conditions.

FIG. 34 show utilization at various temperatures. The utilizationrepresents that corresponding to the theoretical capacity. The examplebatteries 15A & 15B were superior to the comparison battery 5a in termsof the utilization, and differences were remarkable at high temperaturesof 40° C. and 50° C. The example battery 15B offered remarkably higherutilization than that of the comparison battery 15b.

FIG. 35 shows charge curves of positive electrodes in the examplebattery 15A and the comparison battery 15a at 20° C. and 40° C. Thecharge quantity represented by the axis of abscissa indicates thatcorresponding to the theoretical capacity. At 20° C., oxygen evolutionovervoltages of the both batteries 15A, 15a are approximately equal.However, the oxygen overvoltage does not rise even in the final stage ofcharging, in the comparison battery 15a at 40° C. Therefore, it can beunderstood that the charge acceptance is reduced. On the contrary, theoxygen overvoltage rises even in the final stage of charging and a highutilization is obtained even at 40° C. in the example battery 15A. Alsoin the comparison battery 15b, a high utilization is maintained at hightemperature in the same way as the example battery 15A. This is causedby the oxygen evolution overvoltage rise effect owing to Yb ion in theelectrolyte.

As mentioned above, according to the alkaline storage batteries of thisexample, the utilization of the nickel electrode can be increased andthe decrease in the utilization at high temperature can be prevented.

What is claimed is:
 1. A hydrogen absorbing electrode, comprising:ahydrogen absorbing alloy capable of absorbing and releasing hydrogen,said alloy comprising a rare earth element having a basicity weaker thanthat of lanthanum, wherein the alloy is previously subjected to adipping treatment by using an alkali aqueous solution or a weak acidicaqueous solution.
 2. A hydrogen absorbing electrode as set forth inclaim 1, in which the rare earth element is selected from the groupconsisting of samarium, gadolinium, terbium, dysprosium, holmium,erbium, thulium and ytterbium.
 3. A nickel electrode, consistingessentially of:a nickel hydroxide for serving as an active materialcontaining at least one element selected from the group consisting ofcobalt, zinc, cadmium, and magnesium, as a solid solution, wherein thenickel hydroxide is mixed with a rare earth element selected from thegroup consisting of yttrium, holmium, erbium, thulium, ytterbium,europium lutetium, and compounds thereof; and wherein an internal porevolume of the nickel hydroxide is smaller than or equal to 0.1 ml/g. 4.A nickel electrode as set forth in claim 3, in which the compound ofrare earth element is selected from the group consisting of an oxide, ahydroxide and a halogenide.
 5. A nickel electrode, comprising:a nickelhydroxide for serving as an active material having a component accordingto the following equation

    (Ni.sub.1-a X.sub.b Co.sub.c Zn.sub.d)(OH).sub.2,

wherein X, cobalt and zinc are contained as a solid solution, and X isat least one element selected from the group consisting of ytterbium,europium, lutetium and erbium; a=b+c+d; and 0.02≦a≦0.20, 0≦c<0.20,0≦d<0.20.
 6. A nickel electrode, comprising:a nickel hydroxide forserving as an active material mixed with a compound comprising ytterbiumand lutetium and optionally an element selected from the groupconsisting of yttrium, holmium, erbium, and thulium, and a cobaltcompound.
 7. A nickel electrode comprising a nickel hydroxide forserving as an active material mixed witha compound comprising at leastytterbium and lutetium and an element selected from the group consistingof yttrium, holmium, erbium, and thulium in a mixed state or as acomposite compound; and a cobalt compound.
 8. A nickel electrodecomprising a nickel hydroxide for serving as an active material mixedwitha compound comprising ytterbium and lutetium and optionally anelement selected from the group consisting of yttrium, holmium, erbium,and thulium; and a cobalt compound, in which ytterbium and lutetiumlarger than or equal to 35 wt % when converted to a quantity of oxide,and a ratio of a content of ytterbium to contents of ytterbium andlutetium is larger than or equal to 0.75 when converted to the quantityof oxide.
 9. A nickel electrode comprising a nickel hydroxide forserving as an active material mixed withat least one compound consistingessentially of at least one element selected from the group consistingof holmium, erbium, thulium, ytterbium, and lutetium; and a cobaltcompound, in which a percentage of the metallic cobalt is smaller thanor equal to 3 wt % in the selected cobalt compound.
 10. An alkalinestorage battery comprising:a nickel electrode, a negative electrode, aseparator, and an alkaline electrolyte, wherein said nickel electrodeconsists essentially of a nickel hydroxide for serving as an activematerial, wherein an internal pore volume of the nickel hydroxide issmaller than or equal to 0.1 ml/g, and wherein a rare earth element or acompound thereof is coated on a surface of the nickel electrode.
 11. Analkaline storage battery comprising:a nickel electrode comprising anickel hydroxide for serving as an active material, a negativeelectrode, a separator coated on the surface with a compound comprisinga rare earth element, and an alkali electrolyte.
 12. An alkaline storagebattery as set forth in claim 11, in which the coating is carried out atleast on a positive electrode-side face of the separator.
 13. Analkaline storage battery as set forth in claim 10, in which a weightpercentage of coating is 0.1 wt % to 10 wt % relative to a quantity ofthe positive active material.
 14. An alkaline storage battery equippedwith a nickel electrode utilizing a nickel hydroxide for serving as anactive material, a negative electrode, a separator and an alkalielectrolyte wherein,a rare earth element or a compound thereof isdissolved in the alkali electrolyte.
 15. An alkaline storage battery asset forth in claim 14, in which the alkali electrolyte has a principalcomponent of potassium hydroxide and contains sodium hydroxide orlithium hydroxide.
 16. An alkaline storage battery as set forth in claim10, in which the rare earth element is ytterbium.
 17. An alkalinestorage battery as set forth in claim 10, in which the compound of rareearth element is an oxide, a hydroxide or a halogenide.
 18. An alkalinestorage battery as set forth in claim 11, in which a weight percentageof coating is 0.1 wt % to 10 wt % relative to a quantity of the positiveactive material.
 19. An alkaline storage battery as set forth in claim11, in which the rare earth element is ytterbium.
 20. An alkalinestorage battery as set forth in claim 11, in which the compound of rareearth element is an oxide, a hydroxide or a halogenide.
 21. An alkalinestorage battery as set forth in claim 14, in which the rare earthelement is ytterbium.
 22. An alkaline storage battery as set forth inclaim 14, in which the compound of rare earth element is an oxide, ahydroxide or a halogenide.
 23. A nickel electrode, comprising:a nickelhydroxide for serving as an active material mixed with:a compoundcomprising at least ytterbium and lutetium and an element selected fromthe group consisting of yttrium, holmium, erbium, and thulium, and acobalt compound.
 24. A nickel electrode comprising a nickel hydroxidefor serving as an active material mixed witha compound comprising atleast ytterbium and lutetium and an element selected from the groupconsisting of yttrium, holmium, erbium, and thulium; and a cobaltcompound, in which ytterbium and lutetium larger than or equal to 35 wt% when converted to a quantity of oxide, and a ratio of a content ofytterbium to contents of ytterbium and lutetium is larger than or equalto 0.75 when converted to the quantity of oxide.
 25. A nickel electrodeas set forth in claim 3, in which the compound of a rare earth elementis not in solid solution but is in a physically mixed state with theactive material.